1. Field of the Invention
The present invention relates to an automatic focussing apparatus particularly suitable for use in a television camera.
2. Description of the Prior Art
Hitherto, for an automatic focussing apparatus, an apparatus which measures a distance to an object by emitting infrared rays against an object and receiving the reflected infrared rays to measure distance and drives the lens mechanism for focussing, has been known. Such apparatus can measure the distance accurately to the object. However, for moving the lens accurately in accordance with the distance, the lens mechanism for focussing needs to be assembled accurately. The principle of the measure using the infrared rays is shown in FIG. 1. That is, the light emitting diode 101 emitts infrared rays and the lens 102 condenses the infrared rays. The lens 104 condenses the infrared rays reflected by the object 103. The photodiode 105 receives the reflected infrared rays. The position of the photodiode 105 shifts corresponding to the distances to the object as shown by a real line and a broken line in FIG. 1. Accordingly, the distance to the object can be measured by using the position of the photodiode 105, and the lens mechanism for focussing is driven in accordance with the distance. Therefore, the lens mechanism for focussing is required to be assembled with a high precision.
Also, there is a known art for making the lens mechanism for focussing without need of such precision, by using a feedback loop of an automatic focussing apparatus system. Such automatic focussing apparatus system is explained as follows using FIG. 2(a). The rays reflected by the object passes through a lens 1. The image tube 2 converts the photo-signal concerning the object into an electric signal. The pre-amplifier 3 amplifies the electric signal obtained by the image tube 2 and the processing circuit 4 makes .gamma.-correction, blanking process (BLK) and adds a synchronization signal (SYNC), etc. The synchronization signal generator circuit 5 supplies a synchronization signal (SYNC), a blanking signal (BLK), a vertical driving signal (VD) and a horizontal driving signal (HD), etc . . . A deflection circuit 6 deflects an electron beam. A high frequency component detector 7 detects a high frequency wave component from the output signal of the pre-amplifier 3 and, for example, a band-pass filter 7 with center frequency of 1 MHz is used. A motor 12 makes the lens 1, of the lens mechanism for focussing, wobbly by using a signal produced from a given frequency generator circuit which continually changes the state of the focussing of the lens 1. Therefore, the output signal of the band-pass filter 7 includes a component effected by the above-mentioned change of the state of the focussing of the lens 1. A given frequency component detector 10 detects a given frequency component from the output of the band-pass filter 7, the given frequency component resulting from the wobble of the lens 1. A synchronous detector 9 detects a polarity and an amplitude of the given frequency component and adds the output signal to a motor controlling circuit 11. The motor controlling circuit 11 controls the motor 7 responding to the polarity and the amplitude of the signal so that the amplitude of the high frequency component of the output signal of the image tube 2 is made maximum. At this maximum state, the amplitude of the given frequency component is zero, therefore this feedback loop is locked at this maximum state.
The principle of the detecting of a driving direction of the motor 12 is explained as follows, referring to the FIG. 2(b). When the object is at a distance D.sub.1 and the lens 1 is transferred from a position which is in focus for a near object to a position which is in focus for a far object, as compared to the position D.sub.1, the amplitude of the high frequency component of the signal output draws a solid line as shown in FIG. 2(b). In FIG. 2(b), the waves a.sub.1 and a.sub.2 show wobbles of the lens 1 at the given frequency. When the lens 1 is disposed at the position nearer than the position of the actual object at D.sub.1, the given frequency component becomes a wave signal b.sub.1 as shown in FIG. 2(b), and when the lens 1 is disposed at the position farther than the position of the actual object at D.sub.1, the given frequency component becomes a wave signal b.sub.2 as shown in FIG. 2(b). If the motor 12 is determined previously to transfer the lens 1 in the direction indicated by an arrow mark C, when a signal which is obtained by synchronously detecting the wave signal b.sub.1 is supplied to the motor controlling circuit 11, the motor 12 transfers the lens 1 in the direction indicated by the arrow mark C.sub.1. On the other hand, when a signal which is obtained by synchronously detecting the wave signal b.sub.2 is supplied to the motor controlling circuit 11, the motor 12 transfers the lens 1 in the direction indicated by the arrow mark C.sub.2. Therefore, the lens 1 is controlled to come to the position of the distance D.sub.1 where the amplitude of the high frequency component is maximum. As mentioned above, if the lens mechanism for focussing is included in a feedback loop of the automatic focussing apparatus system, the lens mechanism for focussing is not required to be constructed precisely.
However, since the video signal of a television is obtained by scanning, if the given frequency is determined optionally, erronous focussing operation occurs.
Furthermore, if the lens 1 is disposed at a very far position from the position of the object at D.sub.1, the amplitude of the high frequency component becomes very small. Therefore, in this case the amplitude of the given frequency component signal obtained by making the lens 1 wobble is smaller than that of noise and an erroneous operation occurs. Accordingly, when a diaphragm is open at its maximum or around it, a range within which the automatic focussing can be executed is very narrow, therefore the above-mentioned erroneous operation is likely to occur.